Urea constitutes 46% of the worldwide consumption of nitrogen in agriculture and is the most widely used nitrogen fertilizer. However, after its application to soil, its partial hydrolysis to yield ammonia and carbon dioxide occurs. This process is catalysed by the enzyme urease, which is produced by some bacteria and fungi. The gaseous products formed by the hydrolysis reaction (ammonia and carbon dioxide) volatilize to the atmosphere and thus, substantial losses from the total amount of the nitrogen applied to the field occur.
The hydrolysis process can be considerably decelerated by urease inhibitors that are applied together with urea. Among the most effective urease inhibitors are the thiophosphoric triamide compounds disclosed in the U.S. Pat. No. 4,530,714, above all N-(n-butyl)thiophosphoric triamide (NBPT). The use of NBPT was experimentally verified and this compound is now industrially available for the use in agriculture (Watson, C. J. (2005) Proc. Internat. Fertiliser Soc. 454, 1-38).
The thiophosphoric triamides are used also for prevention of nitrogen loss from animal wastes (excrements, manure), which are caused by the enzymatic cleavage of urea present in the wastes to ammonia. According to WO 98/27261, urease inhibitors can be added to animal wastes also for the purpose of decreasing odour formation. Similarly, the urease inhibitors may be utilized in sprays masking animal urine odour, which are used for dissuading animals from undesired territorial behaviour including territory marking by urine.
Industrial grade N-(n-butyl)thiophosphoric triamide (NBPT) is a solid, waxy compound, which decomposes by the action of moisture and elevated temperature. With regard to its consistence, its direct application onto urea particles is very difficult. Technologically more advantageous is the use of the NBPT solutions in a suitable solvent, which should comply with some basic requirements: high solubility and stability of NBPT in the solvent, resistance of the NBPT solutions against the crystallization at a low temperature, low viscosity of the concentrated solutions of NBPT, low toxicity, volatility and flammability, minimum content of water in the commercially available form of the solvent, low cost.
In the U.S. Pat. No. 5,698,003, mixtures of N-(n-butyl)thiophosphoric triamide (NBPT) and aliphatic diols and triols or their esters are described, containing up to 50 wt. % of NBPT, preferably 20-30 wt. % of NBPT. These mixtures are suitable for the impregnation of granular urea, but they are liquid only at the temperatures above 15° C. Below this temperature, they solidify or crystallize. This undesired property renders the manipulation with the solutions and their application to granular urea in the course of its production, particularly during the cold periods of the year, when the temperature can fall deep below 0° C. U.S. Pat. No. 5,698,003 teaches that the addition of so-called “liquid amides” (e.g. 10 wt. % of N-methylpyrrolidone) to these solvents can shift the solidification temperature to 0° C. The liquid amides, however, are health-deleterious (N-methylpyrrolidone is often classified among carcinogenic and teratogenic compounds) and the solidifying temperature achieved is still not sufficient for the manipulation at low temperatures or at freezing.
Another issue is the technical quality and hygroscopicity of the commercially available diols and triols. They often contain few tenths of percent of water and this amount can further increase during storage due to the hygroscopicity of the solvents. During a longer storage period, the presence of water causes the decomposition of N-(n-butyl)thiophosphoric triamide (NBPT) into non-effective substances and is the main cause of the NBPT degradation during a long-term storage. The instability of NBPT towards hydrolysis is well known, e.g. because of it, NBPT is poorly utilizable on moist soils.
Czech patent application PV 2006-422 teaches a solvent system for the preparation of N-alkyl thiophosphoric triamide solutions containing glykolethers, and optionally further auxiliary substances, such as N-methylpyrrolidone or polyvinylpyrrolidone. The glycolethers show excellent solvent properties, but they belong among synthetic solvents. For both ecological and legislative reasons, it is at present more advantageous to use so-called “green” solvents, that decompose to natural substances in the ecosystem.
The above mentioned disadvantages can be solved by liquid compositions of N-alkyl phosphoric or thiophosphoric triamides with a new group of solvents according to the present invention.